1. Field
Embodiments disclosed herein generally relate to plasma etching.
2. Description of Related Art
A plasma reactor for processing a semiconductor wafer typically holds the wafer inside the reactor chamber using an electrostatic chuck (ESC). Plasma ion energy at the wafer surface is controlled by applying a bias voltage to the wafer through the ESC. The ESC essentially consists of an insulator layer having a top surface for supporting the wafer. An electrode or conductive mesh inside the insulator layer beneath the wafer receives a D.C. voltage, creating a voltage drop across the insulator layer between the electrode and the wafer. The voltage drop produces an electrostatic force clamping the wafer to the ESC. The clamping force is determined by the difference between the time-average of the wafer voltage and the D.C. voltage applied to the ESC electrode. The clamping voltage may be accurately controlled (by accurately controlling the D.C. supply voltage) in order to avoid an insufficient clamping voltage or an excessive clamping voltage. An insufficient clamping voltage would allow the wafer to pop off of the ESC. An excessive clamping voltage would increase the current through the wafer to a level that risks damaging the circuit features formed on the wafer surface.
Current flows from the ESC electrode through the dielectric layer to the wafer and returns through the plasma in the chamber. The stronger the clamping force, the greater the conductivity between the wafer and the ESC, and therefore the greater the current through the wafer. In order to accurately control the clamping voltage, the wafer D.C. voltage should be measured accurately. An error in wafer voltage measurement may lead to wafer pop off or to excessive ESC-wafer current.
Use of ESC-wafer contact to control the wafer temperature imposes even more difficulties for accurate control of clamping voltage. For example, the ESC may be heated or cooled so that the wafer is either heated or cooled at a rate determined by the ESC clamping force. The wafer temperature may therefore be accurately set and controlled as a function of the clamping force. In fact, the heat transfer rate may be so great as the clamping voltage is increased, that the wafer temperature may be maintained under much higher heat load than was formerly possible. Wafer bias power may be increased beyond previously permitted levels.
Therefore, there is a need to an apparatus and method for controlling the clamping force.